The present invention relates to microphones, and more particularly to condenser microphone assemblies, such as a backplate with integral spacer made from semiconductor components.
Condenser or capacitance microphones are widely used in the audio, electronics and instrumentation industries. Condenser microphones include a flexible diaphragm or membrane and a rigid backplate that may contain one or more openings. Together, the membrane and the backplate of the microphone form a capacitor, which is also known as a condenser. When a sound wave hits the membrane, the membrane moves, causing a variation in height of the air gap between the membrane and the backplate. This gap variation results in a change in the capacitance of the condenser formed by the membrane and the backplate. If a fixed or controlled charge Q is maintained on the capacitor, a voltage will be formed across the capacitor that will then vary proportionally to the change in the height of the air gap. As is known in the art, conventional diaphragms may be constructed from metal films or metallized polymer films.
For a variety of applications, it is desirable to manufacture small, high quality condenser microphones. As is known in the art, openings in the backplate may be created by drilling or punching holes. Controlling the precise size and location of such holes, which can be critical, becomes more difficult as the holes become smaller.
As is also known in the art, entire condenser microphones, including diaphragms, can be formed on silicon substrates through MicroElectroMechanical Systems (MEMS) fabrication methods, which is the formation of mechanical components based on silicon integrated circuit manufacturing processes. For example, U.S. Pat. No. 5,889,872 discloses a capacitive microphone formed with semiconductor processing techniques. A diaphragm is formed as part of the fabrication by applying a polysilicon layer on a silicon nitride layer. The polysilicon layer is patterned or etched to form a diaphragm.
U.S. Pat. No. 5,870,482 explains challenges associated with maintaining highly compliant and precisely positioned diaphragms fabricated from a silicon wafer. That patent discloses an alternative solid state condenser microphone with a semiconductor support structure.
U.S. Pat. No. 6,075,867 discloses a micromechanical microphone with multiple diaphragms. To address problems of humidity, dust and dirt, the microphone includes two sealing membranes on either side of a transducer. However, an environmental membrane in front of a sensing transducer may affect audio characteristics, such as signal to noise ratio, frequency response, and sensitivity.
The formation of complete condenser microphones through MEMS processing is extremely difficult and expensive. Moreover, condenser microphones constructed entirely from MEMS processing often exhibit inferior audio and reliability characteristics.
The present invention solves many of the aforementioned problems by a microphone assembly comprising a housing, a semiconductor backplate mounted in the housing and a flexible diaphragm located above the backplate. The semiconductor spacer is integrally formed with the backplate and intermediate the backplate and the diaphragm. The backplate and spacer is not integrally formed with the diaphragm, the diaphragm frame, or the housing.
The diaphragm is stretched over and adhesively affixed to the diaphragm frame. The diaphragm frame maintains tension in the diaphragm. The diaphragm is comprised of a metal film or metallized polymer film, and the diaphragm is both a protective environmental barrier and a sensing electrode of a capacitive electroacoustic transducer. The housing may be made of metal, and the backplate made of silicon. The spacer may further comprise an electrically insulating layer, such as silicon dioxide or a fluoropolymer.
The backplate includes a top portion, a bottom portion, and a side portion and a plurality of openings extending from the top portion of the backplate to the bottom portion of the backplate. In one embodiment, the plurality of openings are located along the side portion of the backplate and are radially outward of the spacer. The backplate may be circular, rectangular or another desirable shape. The spacer may consist of an annular wall, a series of arcuate walls, a series of arcuate extensions or a rectangular wall.
The housing comprises an upper lip, and the diaphragm frame comprises a metal ring positioned against the upper lip. The assembly may further comprise a metal contact on the bottom portion of the backplate. Furthermore, the invention may include a spring positioned between the backplate and a lower portion of the housing.
In addition, the invention may comprise a transistor coupled to the housing or the backplate. The microphone assembly may also comprise an application specific integrated circuit (ASIC) coupled to the backplate, and the ASIC may include a transistor.
These as well as other novel advantages, details, embodiments, features and objects of the present invention will be apparent to those skilled in the art from following the detailed description of the invention, the attached claims and accompanying drawings, listed herein, which are useful in explaining the invention.